1. Field of Invention
This invention relates generally to electromagnetic flowmeters, and more particularly to an excitation circuit for supplying periodic current to the field coil of the flowmeter in alternating polarity and to maintain the intensity of the current at a constant level regardless of fluctuations in the supply current source or of other changes which would otherwise affect the intensity level.
2. Prior Art
In an electromagnetic flowmeter such as those disclosed in the Mannherz Pat. No. 4,296,636 and in the Schmoock et al. Pat. No. 4,417,479, the fluid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the flow axis being created by an electromagnet formed by a pair of opposed field coils. When the flowing fluid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the volumetric flow rate, is then amplified and processed in a converter or secondary to provide a flow rate indication.
The magnetic field may either be direct or alternating; for, in either event, the amplitude of voltage induced in the fluid intersecting the field will be a function of flow rate. However, when operating with a direct magnetic flux, the resultant d-c signal current flowing through the fluid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. By alternating the polarity of current applied to the field coils and thereby alternating the resultant field, polarization is rendered negligible, for the resultant signal current and therefore its integral does not build up with time.
In one known electromagnetic flowmeter of this type, current from a supply is fed into the excitation circuit through an extended cable, the circuit including a commutator which acts periodically to reverse the polarity of the current fed to the field coil (or coils). The intensity of the current which flows through the field coil therefore diminishes to an extent depending on the length of the cable. This gives rise to errors in measurement unless the length of the supply cable is taken into account in each installation.
Moreover, fluctuations in the supply current source for the excitation circuit act to vary the intensity of current flowing through the coil. Also, changes in the parameters of the field coil as a result of temperature variations act to vary the intensity of the coil current.